1. Field of the Invention
The present invention relates to a developing apparatus possessing a magnet roller and having a developing roller for feeding a developer to a photoconductor, and a manufacturing method of a developing roller.
2. Description of the Prior Art
The developing apparatus used in a conventional copier comprises an agitation roller for stirring the developer, a developing roller for feeding the developer to a photoconductor, a feed roller for feeding the developer to the developing roller, an ear cutter or developer height regulator for defining the developer depositing on the developing roller, and a toner concentration sensor. Specifically the developing roller consists of, as shown in FIG. 1, a magnet roller 11, a nonmagnetic sleeve 12 disposed on the surface of the magnet roller 11, and others. This magnet roller 11 possesses plural magnetic poles N1, N2, S1, S2, S3 for feeding the developer to the photoconductor 7. These plural magnetic poles individually adsorb and convey the developer, and form magnetic brushes, which are eared states of the developer, in the area near the photoconductor 7. The magnetic flux density distribution is as shown in the diagram. A toner concentration sensor 10 is disposed above between the magnetic poles S1 and N2 in this magnetic flux density distribution.
The toner concentration sensor 10 is a detector of the toner concentration of the developer passing nearby, and various types are known. For example, incorporating a coil, by detecting the fluctuation of the inductance (L) of the coil by the developer passing nearby, the toner concentration of the developer is detected.
This type of toner concentration sensor 10 is, as a matter of fact, influenced by magnetism. That is, if the toner concentration sensor 10 is present within the magnetic field, an error is caused in the output due to the effect of the magnetic field although the toner concentration is the same.
In the conventional toner concentration sensor 10, as shown in the diagram, the magnetic field in which the toner concentration sensor 10 is present is not zero gauss but possesses a specific magnetic flux density, and therefore the toner concentration output is inaccurate. As a result, the toner concentration is controlled on the basis of this inaccurate toner concentration output, and copies of poor picture quality are made.
Meanwhile, as mentioned above, the magnetic flux density distribution is as shown in FIG. 2, and when the ear cutter or developer height regulator 8 for cutting the ear of the developer is disposed between the magnetic poles S1 and N2 in the magnetic flux density distribution, the developer on the surface of the sleeve 12 of the developing roller 6 beneath the developer height regulator 8 is in an eared state because this location is not zero gauss but possesses a specific magnetic flux density. Supposing the humidity or the toner density varies, a difference is caused in the amount of earning of the developer. Therefore, the earning state of the developer is unstable, and the amount of the developer cut off by the developer height regulator 8 and supplied into the photoconductor is not constant. Hence, the quantity of the toner supplied on the photoconductor 7 is not constant.
Moreover since the magnetic poles involve fluctuations as manufacturing errors, the magnetic flux density may be much higher in a region beneath the developer height regulator 8. In this case, the developer ears up higher, and the cutting amount by the developer height regulator 8 differs greatly every time, and the toner quantity supplied on the photoconductor 7 becomes more and more unstable.